The invention relates to defining quadrature-axis magnetizing inductance of a synchronous machine as described in the preamble of claim 1.
Synchronous machines are generally used as motors in applications that require high power and torque. Typical applications are for instance rolling drives in metal industry, in which the power of one synchronous motor can be several megawatts. A rolling drive also needs to be precise and fast to ensure the quality of the final product.
Today, frequency converters are typically used to precisely and quickly control synchronous motors. The motor is controlled with a frequency converter in such a manner that the converter part of the frequency converter produces a desired voltage or current to the motor poles. The most common frequency converter type is a frequency converter with a DC voltage link, in which the output voltages are generated from the direct voltage of the intermediate circuit with an inverter.
By using a frequency converter, it is possible to control a machine quickly and appropriately. So as to make controlling the machine possible during load situations and their changes, an as precise electric model as possible should be drafted of the synchronous machine, and on the basis of the model, the electric state of the machine can be calculated to control the machine. An electric model requires not only a mathematical description of the machine but also information on the machine parameters. These parameters, such as different types of inductances and resistances may be of different size even in similar machines of one manufacturer. The manufacturer of the machine typically declares the parameters required for the machine, but the measured values declared by the manufacturer are often inexact or deficient.
A problem arises especially in defining the magnitude of the quadrature-axis magnetizing inductance of a synchronous machine. Up till now, this has been done by using a load machine, the size of which corresponds to the size of the machine to be measured. Another known alternative is to lock the shaft reliably into place. This problem is especially emphasized in very high power categories, since the prior-art arrangement for defining quadrature-axis magnetizing inductance is significantly large and expensive.